<!DOCTYPE html>
<html>

<head>
    <title>计算属性和侦听器</title>

    <!-- 开发环境版本，包含了有帮助的命令行警告 -->
    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>

    <!-- 因为 AJAX 库和通用工具的生态已经相当丰富，Vue 核心代码没有重复 -->
    <!-- 提供这些功能以保持精简。这也可以让你自由选择自己更熟悉的工具。 -->
    <script src="https://cdn.jsdelivr.net/npm/axios@0.12.0/dist/axios.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/lodash@4.13.1/lodash.min.js"></script>
</head>

<body>
    <!-- 计算属性 -->
    <div id="example">
        <p>Original message: "{{ message }}"</p>
        <p>Computed reversed message: "{{ reversedMessage }}"</p>
    </div>


    <!-- 侦听属性 -->
    <div id="demo">
        {{ fullName }}
        <br>
        {{ fullName2 }}
    </div>


    <!-- 侦听器 -->
    <div id="watch-example">
        <p>
            Ask a yes/no question:
            <input v-model="question">
        </p>
        <p>{{ answer }}</p>
    </div>


    <script>
        // 计算属性
        var vm = new Vue({
            el: '#example',
            data: {
                message: 'Hello'
            },
            computed: {
                // 计算属性的 getter
                reversedMessage: function() {
                    // `this` 指向 vm 实例
                    return this.message.split('').reverse().join('');
                }
            }
        });


        // 侦听属性
        var vm2 = new Vue({
            el: '#demo',
            data: {
                firstName: 'Foo',
                lastName: 'Bar',
                fullName: 'Foo Bar'
            },
            watch: {
                firstName: function(val) {
                    this.fullName = val + ' ' + this.lastName;
                },
                lastName: function(val) {
                    this.fullName = this.firstName + ' ' + val;
                } 
            },
            // 计算属性的 setter
            computed: {
                fullName2: {
                    // getter
                    get: function() {
                        return this.firstName + ' ' + this.lastName;
                    },
                    // setter
                    set: function(newValue) {
                        var names = newValue.split(' ');
                        this.firstName = names[0];
                        this.lastName = names[1];
                    }
                }
            }
        });


        // 侦听器
        var watchExampleVM = new Vue({
            el: '#watch-example',
            data: {
                question: '',
                answer: 'I cannot give you an answer until you ask a question!'
            },
            watch: {
                // 如果 `question` 发生改变，这个函数就会运行
                question: function(newQuestion, oldQuestion) {
                    this.answer = 'Waiting for you to stop typing...';
                    this.debouncedGetAnswer();
                }
            },
            created: function() {
                // `_.debounce` 是一个通过 Lodash 限制操作频率的函数。
                // 在这个例子中，我们希望限制访问 yesno.wtf/api 的频率
                // AJAX 请求直到用户输入完毕才会发出。想要了解更多关于
                // `_.debounce` 函数 (及其近亲 `_.throttle`) 的知识，
                // 请参考：https://lodash.com/docs#debounce
                this.debouncedGetAnswer = _.debounce(this.getAnswer, 500);
            },
            methods: {
                getAnswer: function() {
                    if (this.question.indexOf('?') === -1) {
                        this.answer = 'Questions usually contain a question mark. ;-)';
                        return;
                    }
                    this.answer = 'Thinking';
                    var vm = this;

                    axios.get('https://yesno.wtf/api')
                    .then(function(response) {
                        vm.answer = _.capitalize(response.data.answer);
                    })
                    .catch(function(error) {
                        vm.answer = 'Error! Could not reach the API. ' + error;
                    })
                }
            }
        });

    </script>
</body>

</html>